Hot melt adhesive compositions are used in a variety of applications that require bonding two substrates together including, e.g., packaging applications (e.g., corrugated board and cardboard), bookbinding and footwear manufacturing. A hot melt adhesive composition is generally applied in a molten state and forms a bond as it cools and solidifies. The time required for a hot melt adhesive to cool to the point where it has enough strength to form a bond is referred to as the “set speed.” Set speed is an important parameter in applications that require high speed processing operations such as packaging and book binding operations. For many packaging applications, the bond needs to form rapidly or the resulting package will be poorly sealed or even unsealed.
Waxes such as paraffin waxes, polyethylene waxes, microcrystalline waxes and Fischer-Tropsch waxes are added to hot melt compositions to decrease the viscosity, to decrease the set time or to increase the heat resistance of a hot melt composition. Because waxes are crystalline at room temperature, they tend to cause hot melt compositions formulated therewith to crystallize quickly from the molten stage, which can decrease the set time relative to the composition without the wax. Although this is desirable in some applications, in many applications decreasing the set time will decrease the utility of the hot melt adhesive composition.
Waxes generally are known to detackify pressure sensitive adhesive compositions. In hot melt pressure sensitive adhesive formulations, waxes tend to bloom to the surface of the hot melt composition, which detackifies the composition. For this reason, waxes often are not present in pressure sensitive adhesive formulations.
Wax also tends to crystallize while aging in an elevated temperature environment. When a hot melt adhesive composition is formulated with such a wax, this property tends to impair the ability of the composition to maintain a surface bond to a substrate when exposed to an elevated temperature for an extended period of time.